Hearing aid technology has progressed rapidly in recent years. Technological advancements in this field continue to improve the reception, wearing-comfort, life-span, and power efficiency of hearing aids. With these continual advances in the performance of ear-worn acoustic devices, ever-increasing demands are placed upon improving the inherent performance of the miniature acoustic transducers that are utilized. There are several different hearing aid styles known in hearing aid industry: Behind-The-Ear (BTE), In-The-Ear or All In-The-Ear (ITE), In-The-Canal (ITC), and Completely-In-The-Canal (CTC).
Generally, a listening device, such as a hearing aid or the like, includes a microphone assembly, an amplifier and a receiver (speaker) assembly. The microphone assembly receives vibration energy, i.e. acoustic sound waves in audible frequencies, and generates an electronic signal representative of these sound waves. The amplifier accepts the electronic signal, modifies the electronic signal, and communicates the modified electronic signal (e.g. the processed signal) to the receiver assembly. The receiver assembly, in turn, converts the increased electronic signal into vibration energy for transmission to a user.
The electronic signals generated in the microphone assembly are susceptible to interference, two examples of which are high frequency electromagnetic radiation interference from radio or cell phone transmitters in the range of 1-3 GHz, and power supply noise that is often caused when the receiver (speaker) draws substantial current from the miniature hearing aid battery. This disclosure is directed to the latter interference problem.
The impedance buffer circuit in a miniature electret microphone typically has a power supply rejection (PSR) performance of approximately 26 dB, which for hearing aid applications is considered rather poor immunity to power supply noise. Under noisy power supply conditions, which are quite common in high gain, miniature, hearing aid instruments, this poses a serious problem that is usually addressed by powering the microphone in the hearing aid from voltage regulator electronics having very high PSR. Typical hearing aid voltage regulators have approximately 50 dB of PSR, which improve the effective PSR of the microphone to approximately 75 dB in the hearing aid system. However, achieving this level of PSR in the microphone using a voltage regulator is undesirable for three reasons: it adds the voltage regulator to the bill of materials needed for hearing aid manufacturing, thus increasing the cost of hearing aid manufacture; it increases the power drain on the small hearing aid battery and reduces the battery lifetime; by adding to the number of parts required it makes the hearing aid harder to assemble, as well as taking up precious space within the miniature hearing aid shell.
Limitations of the microphone PSR performance come from limitations of the microphone buffer circuit itself, as well as from inter-trace stray capacitance limitations associated with the hybrid circuit. Since a typical electret transducer has a source capacitance on the order of 2 picoFarads (10−12 F), 60 dB of PSR requires that these inter-trace stray capacitances from the buffer circuit input to the power supply remain one-thousandth of this or smaller, i.e. on the order of a femtoFarad (10−15 F) or less. Reduction of inter-trace stray capacitance dramatically improves the performance of the overall listening device.